1. Field of the Invention
The present invention relates to a process for producing polyester resins including dicarboxylic acid constitutional units and diol constitutional units having a cyclic acetal skeleton, which are excellent in mechanical properties, particularly in impact resistance and have a low yellowness.
2. Description of the Prior Art
Polyethylene terephthalate (hereinafter referred to as “PET”) has been extensively used in various applications such as films, sheets and hollow containers because of its excellent transparency, mechanical properties, melt stability, solvent resistance, fragrance retention and recyclability. However, being not necessarily good in heat resistance, the modification of PET has been widely made by copolymerization.
It has been known that the heat resistance, adhesion property, flame retardancy, etc. of polymers can be improved by the modification with a compound having a cyclic acetal skeleton. Such improvements are attributable to the rigid skeleton of cyclic acetal and the acetal bond. Therefore, there have been proposed various modifications of polyester resins by copolymerizing a compound having a cyclic acetal skeleton.
For example, U.S. Pat. No. 2,945,008 teaches that PET modified with 3,9-bis (1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane exhibits a high glass transition point and is excellent in heat resistance. Japanese Patent 2971942 discloses a container excellent in transparency and heat resistance which is made of a copolyester produced from terephthalic acid, 1,4-butane diol and a glycol having a cyclic acetal skeleton and a production method thereof. Japanese Patent 1979830 discloses a polyester excellent in heat resistance and transparency which is produced using a diol having a cyclic acetal skeleton.
Regarding the adhesion property attributable to the acetal bond, Japanese Patents 1843892, 1855226 and 1902128 and JP 4-88078 A disclose adhesives, adhesive compositions and coating agents which contain polyesters produced using a diol or dicarboxylic acid having a cyclic acetal skeleton.
As other polyesters produced using a dicarboxylic acid or diol having a cyclic acetal skeleton, there have been proposed a polyester combined yarn with different shrinkage (JP 3-130425 A), a modified polyester film (JP 8-104742 A), a biodegradable polyester (JP 9-40762 A), an electrostatic developing toner (Japanese Patent 1652382), and a flame retardant resin composition (JP 2000-344939 A).
Polyester resins are generally produced by a direct esterification method or a transesterification method. In the direct esterification method, a dicarboxylic acid is esterified into a bishydroxyalkyl dicarboxylate by an excess of diol, which ester is then polycondensed into polyester under reduced pressure. In the transesterification method, an ester produced from a dicarboxylic acid and a monohydric alcohol is transesterified into a bishydroxyalkyl dicarboxylate by an excess of diol, which ester is then polycondensed into polyester under reduced pressure. In the production of PET, the direct esterification method is industrially advantageous over the transesterification method because terephthalic acid is less expensive than dimethyl terephthalate. Since the esterification of a dicarboxylic acid with a diol needs no catalyst, a high-quality polyester resin containing a less amount of impurities such as catalyst residues is produced in the direct esterification method, this making the direct esterification method further advantageous over the transesterification method. In the production of ester of a dicarboxylic acid with a diol, alcohol is by-produced in the transesterification method, whereas water is by-produced in the direct esterification method. Therefore, the direct esterification method puts a less load on the environment as compared with the transesterification method. For these reasons, at present, the direct esterification method has been predominantly used in the industrial production of PET.
However, it has been found that the production of a polyester resin containing diol constitutional units having a cyclic acetal skeleton (hereinafter referred to as “cyclic acetal diol-modified polyester resin”) by a common direct esterification method involves some drawbacks. In such a production, the cyclic acetal skeleton is decomposed to form three- and four-functional substances by the action of carboxyl groups present in the reaction system and generated water, thereby broadening the molecular weight distribution of resultant polyester resins or provide gelated products. Thus, the cyclic acetal diol-modified polyester resins produced by the direct esterification method are considerably poor in moldability, mechanical properties, etc. as compared with compositionally analogous polyester resins produced by the transesterification method.
Among the above patent documents related to cyclic acetal diol-modified polyester resins, only JP 3-130425 A describes the direct esterification method. However, actual production conditions and the ring opening of the cyclic acetal skeleton by the action of dicarboxylic acid are not disclosed therein.
Under recently increasing attraction to recycling of PET, especially chemical recycling thereof, many chemical recycling methods have been proposed (JP 2002-60543 A, JP 2002-60369A and JP 2002-167469 A). The method for producing polyester resins using bis(β-hydroxyethyl) terephthalate (hereinafter occasionally referred to merely as “BHET”) recovered from PET by chemical recycling method puts a light load on the environment and is industrially advantageous. Since substantially free from acid ends derived from the dicarboxylic acid, BHET is suitable as the raw material for the cyclic acetal diol-modified polyesters. However, none of the above patent documents discloses the method of producing the cyclic acetal diol-modified polyesters from a bishydroxyalkyl dicarboxylate, which is recovered by depolymerizing polyester resins, as the raw material.
The inventors found a process of producing cyclic acetal diol-modified polyester resins, which comprised a step of esterifying a dicarboxylic acid into a bishydroxyalkyl dicarboxylate or its polymer with a diol having no cyclic acetal skeleton, a step for transesterifying the ester groups with a diol having a cyclic acetal skeleton, and a step for polycondensing the resultant ester. The inventors further found that, in the above process, it was important to control the carboxyl group concentration and the water content in the reaction system during the transesterification of the bishydroxyalkyl dicarboxylate or its polymer with the diol having a cyclic acetal skeleton, and found that, by such a control, the decomposition of the cyclic acetal skeleton and the change of the cyclic acetal diol into three- and four-functional substances were prevented and the polyester resins were stably produced without causing gelation and without broadening the molecular weight distribution. On the basis of these findings, the inventors filed a Japanese patent application which was published as JP 2004-137477A. This application was directed also to a process of producing cyclic acetal diol-modified polyester resins wherein BHET putting a light load on the environment was used as the raw material. However, the clogging of pipes of production apparatus due to sublimable diol having a cyclic acetal skeleton as well as the mechanical properties and yellowness of the cyclic acetal diol-modified polyester resins produced by the proposed method are not considered therein.